After more than a dozen years of basic research into the submillimeter and far infrared range, THz wave research has finally come into its own, and is recognized by the world scientific community as a new frontier. While femtosecond laser pumped THz wave sources have opened up a new vista in applied research, the ideal THz wave source will likely require high temporal and spatial coherence. When this level of quality is finally made available in a user-friendly device, there is little doubt that applied research efforts into the THz region will enjoy a true renaissance.

In this direction we have developed a widely tunable (0.7-2.4 THz, 125-430 µm, 23-80 cm^-1) injection seeded THz-wave parametric generator (IS-TPG) that operates at room temperature. The spectral resolution (<100 MHz, 0.003 cm^-1) is the Fourier transform limit of the nanosecond THz wave pulses. The continuous scanning and the narrow spectral bandwidth of the IS-TPG are verified in absorption spectra of low-pressure water vapor. The output has a high peak power (>200 mW) and a small beam divergence, which are suitable for applications such as spectroscopic imaging.

While airports and factories have used X-ray imaging for inspection, human safety has always been a concern. THz imaging, on the other hand, is safe, and its application in food inspection and other uses should not meet with objections from customers. THz waves can penetrate plastics, semiconductors, cardboard, wood, rubber, textiles, fat, and dry foods, making them suitable for inspection tasks involving products made from these materials.

In our laboratory, THz waves continue to broaden their range of applications: Illicit drugs hidden in envelopes are detected using spectral fingerprints, the seal of flexible plastic packages are checked for the presence of micro-leaks, the water content in plants is monitored for agricultural purposes, and integrated circuits are tested for electrical failures by the laser-terahertz emission microscope.

1. Non-destructive THz imaging of illicit drugs using spectral fingerprints
The absence of non-destructive inspection techniques for illicit drugs hidden in mail envelopes has resulted in such drugs being freely smuggled across international borders. We have developed a novel basic technology for THz imaging, which allows detection and identification of drugs concealed in envelopes by introducing the component spatial pattern analysis. The spatial distribution and the composition of the targets are obtained from THz multispectral transillumination images, using absorption spectra measured with a tunable THz wave source. In our first tests the samples were methamphetamine and MDMA, two of the most widely consumed illegal drugs in Japan, and aspirin as a reference.

2. Laser-Terahertz Emission Microscope (LTEM) for semiconductor device testing
We proposed the LTEM as a technique for inspecting electrical faults in integrated circuits (IC). Commercial IC's were tested during operation. Clear two-dimensional THz-emission images of the IC chip are recorded as the chip is scanned with a femtosecond laser beam. The LTEM images of damaged chips show different patterns from those of normal chips. Also, by modulating the input signal of the IC chip, the signal lines can be traced. These results suggest that the LTEM could become a useful tool for inspecting and monitoring IC quality.